This is a Python-Fu extension/script for GIMP to scale JPEG images in batch mode from the command line.
The script expects a path to a directory where the images are located.
When for example your images are in /tmp/images/ and you put the script gimpbatchfuncs.py below into a file
then you can call from the directory containing gimpbatchfuncs.py

> gimp -idf --batch-interpreter=python-fu-eval -b 'import sys; sys.path=["."]+sys.path;import gimpbatchfuncs; gimpbatchfuncs.scale_images_dir("/tmp/images/")'

Every JPEG image in that directory will be scaled and saved with a new name, which is 
the old name appended with a 'g' just before the file extension. So abc.jpg becomes abcg.jpg and so on.

To summerize:

1. Put your images in a directory, say /tmp/images
2. Put gimpbatchfuncs.py somewhere and from there call the command above with "/tmp/images/" 
   as parameter to gimpbatchfuncs.scale_images_dir()

The command explained:

   gimp -idf --batch-interpreter=python-fu-eval     # To run gimp in Python-Fu mode
   -b 'COMMANDS'                                    # What follows -b are the COMMANDS (in single quotes) to run 
                                                    # which are:
   import sys;                                      # import sys
   sys.path=["."]+sys.path;                         # add the current directory to the search path
   import gimpbatchfuncs;                           # so we can import our script (note it's without the .py ending)
   gimpbatchfuncs.scale_images_dir("/tmp/images/")' # and call the scale function with the image directory as parameter

In the gimpbatchfuncs.py script there are two variables to control how the images are scaled:

final_size           # Gives the size (height or widht, which is controlled by fianl_size_is_height) of the scaled image
fianl_size_is_height # If set to True, final_size is the height of the scaled image
                     # If set to False, final_size is the width of the scaled image

So, if you want images to be 200 pixels in width. Then you set

final_size = 200
final_size_is_height = False

------------------------------------------------- gimpbatchfuncs.py -----------------------

#!/usr/bin/python
import os
from gimpfu import *

# size in pixels of the scaled image
final_size = 800

# if set to True, final_size is the height of the new image
# if set to False, final_size is the width of the new image
final_size_is_height = True

# scale one jpg image and save it.
def scale_one_image(path):
   image = pdb.file_jpeg_load(path, "")
   drawable = pdb.gimp_image_get_active_drawable(image)
   if final_size_is_height:
      f=1.0*drawable.height/final_size
      nw=int(round(drawable.width/f))
      nh=int(final_size)
   else:
      f=1.0*drawable.width/final_size
      nw=int(final_size)
      nh=int(round(drawable.height/f))
   pdb.gimp_image_scale(image, nw, nh)
   nn=os.path.splitext(path)[0]+"g.jpg"
   pdb.file_jpeg_save(image, drawable, nn, "",0.95, 0, 0,0, "", 0, 1, 0, 0)

# scale only one jpg image, with hard-coded path. for testing.
def test():
   scale_one_image("/tmp/test.jpg")
   pdb.gimp_quit(TRUE)

# scale all jpg images in directory 'path'. Scaled images will be saved there with 'g' appended to their name.
def scale_images_dir(path):
   if not os.path.isdir(path):
      print("Argument", path, "is not a path. Exiting.")
      pdb.gimp_quit(TRUE)
      return
   path = os.path.abspath(path)
   files = os.listdir(path)
   while len(files) > 0:
      scale_one_image(path+"/"+files.pop())
   pdb.gimp_quit(TRUE)